1. Field of the Invention
The present invention relates to a multifilament type plastic optical fiber having in the cross-section thereof 10 to 10,000 islands having light-transmitting characteristics, which is valuable as an image-transmitting optical fiber or a light guide.
2. Description of the Related Art
A multifilament type plastic optical fiber is advantageous over a multifilament type optical fiber comprising glass type filaments in that the flexibility is high, the handling property is good and individual filaments constituting the multifilament type optical fiber are not broken by bending. Accordingly, development and utilization of multifilament type plastic optical fibers have been attempted. For example, the processes disclosed in U.S. Pat. No. 3,556,635 and Japanese Unexamined Patent Publication No. 56-39505 are known.
The multifilament type plastic optical fiber disclosed in U.S. Pat. No. 3,556,635 is prepared according to a process in which spinning is carried out in a spinning apparatus as shown in FIG. 1 of this U.S. patent, to which a spinning nozzle having a structure as shown in FIG. 2 of this U.S. patent is attached. Accordingly, this multifilament type plastic optical fiber has an angular cross-section in which light-transmitting islands having a substantially rectangular cross-section are arranged, as shown in FIG. 3 of the U.S. patent.
This multifilament type plastic optical fiber is defective in that since the cross-section of the light-transmitting islands exerting the light-transmitting function is rectangular, the light transmission characteristics are insufficient.
Japanese Unexamined Patent Publication No. 56-39505 discloses a process for the preparation of a multifilament type plastic optical fiber, and it is taught that a spinneret disclosed in Japanese Unexamined Patent Publication No. 54-116417 is used for the production of this multifilament type plastic optical fiber. The sectional structure of this spinneret is as shown in FIG. 3 of the accompanying drawings. Referring to FIG. 3, according to this known process, a core-forming polymer is supplied from 31, a sheath-forming polymer is supplied from 32 and a sea-forming polymer is supplied from 33, a three-layer true conjugate yarn is spun out from 34 and gathered and integrated in a gathering hole 36 of a gathering orifice plate 34 to obtain a multifilament type plastic optical fiber. According to this process, if the occupancy ratio of the sea in the cross-section of the multifilament type plastic optical fiber is at least 40%, that is, the number of the light-transmitting islands is small, the islands are relatively uniform in the cross-sectional shape, but the element density of the image transmitted by this optical fiber is low and no practically good result can be obtained. If a multifilament type plastic optical fiber in which the occupancy ratio of the seal is lower than 10% is prepared according to this process, the cross-section of the islands present in the peripheral portion of the cross-section of the optical fiber is tetrasonal or heptagonal, as shown in FIG. 4 of the accompany drawings, but islands present in the interior of the cross-section of the optical fiber have heptagonal to nonagonal cross-sections. Transmission of an image is substantially impossible by this optical fiber, and the respective islands constituting the multifilament type plastic optical fiber are different from one another in the light-transmitting characteristics and the light quantity unevenness over the entire section is inevitably large.